A large spam campaign observed on Twitter during the last couple of days directed users to malicious websites that exploited vulnerabilities in browser plug-ins to infect their computers with rogue antivirus programs, according to security researchers from antivirus firm Kaspersky Lab.

While monitoring the spam campaign for several hours, Kaspersky's researchers counted 4,148 tweets containing malicious links being sent from 540 compromised Twitter accounts.

"Our analysis is just a snapshot at a given time, and is lower than reality," said Kaspersky Lab senior malware researcher Nicolas Brulez in a blog post on Wednesday.

The rogue tweets contained messages such as "online virus check," "proven anti-virus," "excellent anti-virus," as well as links to websites with .TK and .TW1.SU domain names.

The high variation of links, messages and hijacked accounts used in this spam campaign could explain why Twitter's automated spam filters weren't successful at blocking it.

In addition to traditional rogue scareware pages that use fake antivirus alerts to trick users into downloading the malicious programs, this campaign also used the BlackHole exploit toolkit to infect computers without user interaction.

BlackHole is a popular tool used by cybercriminals to perform so-called drive-by download attacks -- attacks that exploit vulnerabilities in older versions of browser plug-ins like Java, Flash Player or Adobe Reader, to install malware.

Even though scareware is not as prevalent these days as it was a couple of years ago, sophisticated attack campaigns like this one, which require significant resources to pull off, suggest that such applications still play an important role in the cybercriminal ecosystem. Scareware has always been a good way of funding other malicious operations that are not as profitable.

As always, users should keep all software installed on their computers, especially the browsers plug-ins, up-to-date, in order to reduce their chances of being affected by drive-by download attacks.

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